According to conventional methods for improving soft ground, the upper surface of soft ground to be improved (hereinafter referred to as ground to be improved) is coated with an airtight sheet, and the above described ground to be improved is then subjected to vacuum pressure, so that an area of reduced pressure which is separated from the peripheral area of the ground to be improved is created in the above described ground to be improved, and an embankment is laid on the soft ground, and thus, the load of the embankment having high density is applied, and thereby, the soft ground is converted to hard ground.
Concretely, as shown in FIG. 17, vertical draining members 1 are driven into ground to be improved A at predetermined intervals, and then, a horizontal drain 2 is placed on top so as to make contact of the upper end portions la of these vertical draining members 1, and a water collecting pipe 3 which is linked to a vacuum pump 5 is connected to this horizontal draining member 2 via a vacuum tank 4, and furthermore, the upper surface of the ground to be improved A is coated with an airtight sheet 7, together with the upper end portions la of the above described vertical draining members 1, the horizontal draining members 2 and the water collecting pipe 3. After this, the vacuum pump 5 which is connected to the above described water collecting pipe 3 via the vacuum tank 4 is operated.
As a result, vacuum pressure is conveyed from the vacuum pump 5 to the ground to be improved A via the horizontal draining member 2 and the vertical draining members 1 so that the ground in areas surrounding each vertical draining member 1 with the vertical draining members 1 at the center is converted to regions of reduced pressure (hereinafter referred to as reduced pressure regions).
The vacuum pressure is conveyed from the ground surrounding the vertical draining members 1, which has become reduced pressure regions, to surrounding ground further on the outside, and as a result of this, pressure is created in the ground (water pressure, soil pressure) toward the vertical draining members 1.
Pore water included in the ground surrounding the vertical draining members 1 is sucked toward the vertical draining members 1 when this pressure is created in the ground, and water is discharged through the vertical draining members 1, the horizontal draining member 2 and the water collecting pipe 3, which form a water discharging path, and together with this, the ground further around the outside of the ground around the vertical draining members 1 also becomes reduced pressure regions.
In this manner, reduced pressure regions spread to the ground surrounding the vertical draining members 1 with the vertical draining members 1 at the center, and shortly thereafter, the entire region of the ground to be improved A becomes a reduced pressure region, and at the same time, the density and strength increase with the vertical draining members 1 at the center, so that the density and the strength increase in the entire region of the ground to be improved A.
As described above, the ground is converted to hard ground while a mound 6 is created on top of the airtight sheet 7, and thereby, the ground to be improved A is pressed by the load of the created mound 6, so that the density increases and water is removed, and the above described suction of water due to the difference in pressure works together, and thus, the density increases and the ground to be improved A settles (see Patent Document 1).    Patent Document 1: Japanese Patent No. 3270968 (see claims 1 and 2, as well as FIG. 7)